May 16, 2013

Toba eruption did not cause volcanic winter in Africa ~75ka

Ash from the Toba supereruption in Lake Malawi shows no volcanic winter in East Africa at 75 ka

Christine S. Lane et al.

The most explosive volcanic event of the Quaternary was the eruption of Mt. Toba, Sumatra, 75,000 y ago, which produced voluminous ash deposits found across much of the Indian Ocean, Indian Peninsula, and South China Sea. A major climatic downturn observed within the Greenland ice cores has been attributed to the cooling effects of the ash and aerosols ejected during the eruption of the Youngest Toba Tuff (YTT). These events coincided roughly with a hypothesized human genetic bottleneck, when the number of our species in Africa may have been reduced to near extinction. Some have speculated that the demise of early modern humans at that time was due in part to a dramatic climate shift triggered by the supereruption. Others have argued that environmental conditions would not have been so severe to have such an impact on our ancestors, and furthermore, that modern humans may have already expanded beyond Africa by this time. We report an observation of the YTT in Africa, recovered as a cryptotephra layer in Lake Malawi sediments, >7,000 km west of the source volcano. The YTT isochron provides an accurate and precise age estimate for the Lake Malawi paleoclimate record, which revises the chronology of past climatic events in East Africa. The YTT in Lake Malawi is not accompanied by a major change in sediment composition or evidence for substantial temperature change, implying that the eruption did not significantly impact the climate of East Africa and was not the cause of a human genetic bottleneck at that time.

6 comments:

"The YTT in Lake Malawi is not accompanied by a major change in sediment composition or evidence for substantial temperature change, implying that the eruption did not significantly impact the climate of East Africa and was not the cause of a human genetic bottleneck at that time".

And the survival of the 'Hobbits' in Flores suggests it had a minor influence even near the eruption centre. I think people just like the idea of natural disasters. A hangover from 'end of the world' religious beliefs?

Well, -1.5 C may not be significant in East Africa, but it can be in climates that are already marginal (and where the cooling might have been more extreme).

But, more importantly, this does not change the fact that climate ooA deteriorated during and after this time, and soon also became much dryer - making the time period ~70,000 - 55,000 ya one of the worst possible for a successful ooA or out of Arabia movement.

[Stanley] Ambrose was . . . the first to suggest that Toba could have been responsible for human differentiation, producing the various “races” and other signs of major biological and cultural diversity. However, if all humans were confined to Africa when Toba hit, as Ambrose assumes, it's impossible to see how any differences produced by that event could have evolved into the worldwide distinctions so evident today, of which Africa is a relatively homogeneous part. Toba can explain the large-scale differentiation patterns we now see only if humans had already left Africa and had occupied most or all of the south Asiatic coast by the time it erupted. However, Ambrose, for reasons that continue to puzzle me, insists that this is not possible and that all the archaeological evidence points to an African exodus after the Toba eruption, not before (personal communication).

"The YTT in Lake Malawi is not accompanied by a major change in sediment composition or evidence for substantial temperature change, implying that the eruption did not significantly impact the climate of East Africa"... okay fine...."and was not the cause of a human genetic bottleneck at that time." Does not follow.

Exactly, Wil. The evidence appears to rule out a bottleneck in Africa, but says nothing about a bottleneck centered in South Asia, as posited by Stephen Oppenheimer -- and suggested (though not confirmed) by Petraglia's research.

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